Blow molding is a commonly used technique to produce a variety of thin walled containers. Unlike injection molding, blow molding does not rely on very high pressure to form the finished part. Instead, a parison is provided in a mold and inflated or blown. Air between the parison and the mold escapes the mold through venting channels. Since the pressure used is low in comparison to injection molding, the mold is relatively inexpensive. This procedure is used to make a wide variety of plastic containers such as plastic bottles and pails.
A variety of prior art exists that describes different ways of producing different blow molded shapes. For example, in U.S. Pat. No. 4,724,116 to Aoki et al., a parison having a varying thickness is used to produce a blow-molded bottle. The bottle has a substantially elliptical cross section. Thus, the parison used in this process features thin walls in the region where more expansion is desired and thicker walls where less expansion is desired.
A person of skill in the art of blow molding, after having reviewed Aoki et al. might conclude that a substantially conical shape would be produced by varying the thickness of the parison as a function of parison length. Although this will work to an extent, it is flawed in that the thin section of the parison is most easily expanded, and consequently, if the thin section is too thin relative to the thick section the parison will fail by rupturing prior to taking the shape of the blow mold. This failure represents a waste of time and material.
Alternatively, a parison with consistent thick walls is used. Although this technique will work to an extent, it results in a container having walls with different thickness. Ideally, a less wasteful container will have uniformly thin walls regardless of the shape of the container. This saves material and consequently reduces operating costs.
Additionally, many shapes with a substantial variation in radius have at least one open end as well. Typically, producing such a shape involves blowing a sealed container shape and cutting away the openings. As the area of the opening is quite large, material that is cut away represents a substantial portion of the original parison.
It would be advantageous to provide a new technique of extrusion blow molding that produces even wall thickness over areas of differing radii while keeping the costs of the parts and the tools to a minimum. Additionally, it would be beneficial if such a technique supported shapes with a large opening without the operations associated with cutting away material.